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    SubjectRe: [PATCH 01/25] x86: Make SMAP 64-bit only
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    On March 18, 2019 11:10:22 AM PDT, Linus Torvalds <torvalds@linux-foundation.org> wrote:
    >On Mon, Mar 18, 2019 at 10:51 AM Peter Zijlstra <peterz@infradead.org>
    >wrote:
    >>
    >> How about I do a patch that schedules EFLAGS for both 32bit and
    >64bit,
    >> mark this for backporting to infinity.
    >>
    >> And then at the end, after the objtool-ac bits land, I do a patch
    >> removing the EFLAGS scheduling for x86_64.
    >
    >Sounds sane to me.
    >
    >And we can make it AC-conditional if it's actually shown to be visible
    >from a performance standpoint.
    >
    >But iirc pushf/popf isn't really that expensive - in fact I think it's
    >pretty cheap when system flags don't change. Which would be the common
    >case unless you'd also make the popf do the irq restore part and
    >simply make finish_lock_switch() re-enable irq's by doing an
    >irqrestore?
    >
    >I think popf is like 20 cycles or something (and pushf is just single
    >cycles). Probably not worth worrying about in the task switch context.
    >
    > Linus

    Yes, the problem isn't scheduling per se but the risk of hiding problems.
    --
    Sent from my Android device with K-9 Mail. Please excuse my brevity.

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